The present invention relates generally to fuel assemblies for nuclear reactors and, more particularly, is directed to a nuclear fuel rod having an improved end plug for sealing a pressurized gas within the fuel rod.
As is well known in the art, the overall efficiency of a nuclear reactor can be increased and the useful life of its fuel rods prolonged if the fuel rods are internally pressurized. The concept of pressurized fuel rods is set forth in U.S. Pat. No. 3,677,894, granted to Harry M. Ferrari, which discloses a fuel rod having a thermally-decomposable body located therewithin, which when heated, generates a gas to internally pressurize the fuel rod.
Another manner of pressurizing fuel rods is during fabrication thereof wherein one of its end plugs is provided with an inlet gas orifice which is subsequently welded closed after the fuel rod has been filled with an inert-gas under pressure. One type of such an end plug can be seen in U.S. Pat. No. 4,075,454, granted to Duncan et al. The Duncan et al. end plug has a radially extending, gas inlet orifice which leads into a central machined out area of the plug (cavity) for direct flow communication with the inside of the fuel rod. After the fuel rod is filled with a pressurized gas, the inlet orifice is sealed closed with a weld puddle as the end plug is welded on the end of the fuel rod. Besides the costly machining of the central cavity in the plug, it is very difficult to seal the inlet orifice due to peel back problems. Additionally, such seal requires a time consuming and expensive inspection operation to insure that the seal is proper. Another type of end plug can be seen in the German No. 1,238,589 patent which appears to show a flat on the reduced diameter portion of the end plug for rod pressurization followed by a girth seal weld. Similar in construction and operation to the German end plug, the inventor is aware of an end plug having its reduced diameter portion provided with four small, circumferentially spaced, longitudinally extending grooves that extend the full axial height of the plug's reduced diameter portion. An end plug which has a "pressurization" flat or grooves extending the entire length of its reduced diameter is very difficult to seal by girth welding, yet alone, to obtain a proper seal that insures against leakage. Rather than having complete metal-to-metal contact of the end plug with the end of the fuel tube, the "flat" or "grooves" define a void, and thus, during the welding operation, there is no place for the heat to dissipate which results in the tube wall melting back ("peel back").
Yet another, and one of the most common end plug constructions used for pressurizing a fuel rod during the fabrication stage can be seen in Boyko et al. U.S. Pat. No. 4,003,788 which shows an end plug provided with a small axial bore. In such an arrangement, the end plug is first hermetically sealed by girth welds on the end of the tubular cladding, the rod is then placed in a pressurized weld chamber where the gas under pressure enters into the rod through the axial bore, and then the bore opening is fused closed, such as, by one or another one of the methods described in U.S. Pat. Nos. 3,683,148; 3,725,635; or 3,842,238. The method generally used for closing this gas inlet opening (passageway) in the end of the plug is the well known TIG (Tungsten Inert-Gas) process wherein the opening is sealed by a short duration (approximately 2 seconds) weld pulse that produces a fusion weld across the opening. During this welding operation, the molten material is propelled into the passageway by the arc "blast" to produce a nugget sealing the end and extending into the passageway, often as a series of droplets. It is very difficult to insure that what appears as adequate penetration is indeed weld of high integrity fused to the wall of the passageway. Inspection of the seal weld by non-destructive means is difficult and time consuming, and the expensive state of the art inspection devices required are of marginal reliability and effectiveness. Additionally, the close proximity of the electrode and the turbulence created by the arc has the demonstrated potential to contaminate the weld with diffused tungsten which has been found in recent studies to be a factor in predicting cycle coolant activity.
Consequently, there exists a need for an end plug for fuel rod pressurization during the fabrication stage which not only provides for metal-to-metal contact of the plug with the end of the fuel tube to thereby eliminate any possible peel back problems when applying a girth weld, but also, an end plug that does not require an axial bore orifice to fill the rod with a pressurized gas and the many problems associated therewith in obtaining a proper seal.